In semiconductor processing, a large portion of the yield losses can be attributed to contaminations by particles and films of various nature. The contaminants may be organic or inorganic particles, films formed of polymeric bases, or other ionic based materials. The particles or films may be generated as byproducts in the reaction of reactant gases, by the surrounding environment, by the processing equipment or by the handling of manufacturing personnel. Some contaminants are particles or films generated from condensed organic vapors, solvent residues, photoresist or metal oxide compounds.
Typical problems and the detrimental effects caused by particle or film contaminants are poor adhesion of deposited layers, poor-formation of LOCOS oxides, or poor etching of the underlying material. The electrical properties and the stability of devices built on the semiconductor substrate may also be seriously affected by ionic based contaminants. The various forms of contaminants therefore not only reduce the product yield but also degrades the reliability of the devices built. For instance, contaminant particles can cause a device to fail by improperly defined patterns, by creating unpredictable surface topography, by inducing leakage current through insulating layer, or otherwise reducing the device lifetime. It is generally recognized that a particle contaminant that exceeds one-fifth to one-half of a minimum feature size on a device has the potential of causing a fatal defect, i.e. a defect that causes a device to fail completely. A defect of smaller size may also be fatal if it falls in a critical area, for instance, a particle contaminant in the gate oxide layer of a transistor. In modern high density devices, such as a dynamic random access memory chip, the maximum allowable number of particle contaminants per unit area of the device must be reduced accordingly in order to maintain an acceptable yield and reliability.
One of the widely used processing techniques for semiconductor wafers is a low pressure chemical vapor deposition (LPCVD) technique can be carried out in an apparatus such as that shown in FIG. 1. The LPCVD method has been widely used in the deposition of silicon nitride or TEOS oxide films on semiconductor wafers. In the method, a gas containing the structural elements of the film material to be formed is first fed into a chamber, followed by heating the gas mixture to induce a chemical reaction to deposit the desired film on the semiconductor substrate. In a conventional CVD method, a silicon nitride film can be deposited by a chemical reaction between silane (SiH.sub.4) and ammonia (NH.sub.3) at 1 ATM and a temperature of 700.about.900.degree. C., or by a mixture of dichlorosilane (SiCl.sub.2 H.sub.2) and ammonia at a reduced pressure and a temperature of 700.about.800.degree. C.
As shown in FIG. 1, reactant gases of dichlorosilane 12 and ammonia 14, each carried by a carrier gas of nitrogen, are fed into the process chamber 18 through the inner tubes 40. The reaction gases are mixed at the bottom portion of inner tubes 40. Manifold 16 provides inlets and outlets for the gases and is used as a pedestal support for the inner tubes 40 and the outer tubes 24. The process chamber 18 is first evacuated by vacuum pump not shown prior to the reaction. A purge gas of nitrogen 22 is then used to fill the process chamber 18 and to drive out any residual gas left from the previous deposition cycle. A cold trap 26 maintained at sub-ambient temperature, e.g., at approximately 12.about.18.degree. C., is used in the vacuum line to trap particles that cannot be pumped away. The manifold 16 is provided with a pressure sensor 28 which is connected via a conduit 30 to the manifold 16 at a pressure port 32. A main valve 34 and pressure switches 20 are provided in the vacuum evacuation line for controlling the fluid flow. A vent line 48 is connected to the vacuum evacuation line for venting spent reactant gases through control valves 52 and 54 to the exhaust vent 56.
After the reactant gases of SiCl.sub.2 H.sub.2 and NH.sub.3 are mixed in the inner tube 40, the gas mixture is flown into the process chamber 18 to deposit silicon nitride films on wafers held in a wafer boat (not shown). It has been observed that during the reaction between SiCl.sub.2 H.sub.2 and NH.sub.3, a reaction byproduct of NH.sub.4 Cl is frequently produced. The ammonium chloride powder which is in a very fine powdery form causes a defect on the wafer surface known as nitride haze. It is believed that during a niride deposition process, contaminating powder may be coated inside the conduit between the chamber 18 and the cold trap 26, inside the conduit between the cold trap 26 and the gate valve 34, inside the gate valve 34, and inside the conduit between the gate valve 34 and the automatic pressure controller 20. The powdery contaminant may then be siphoned back into the process chamber 18 during an unintentional back-flow process. The fine powder of ammonium chloride deposits on top of a wafer surface and forms a haze defect. The nitride haze, once formed, is very difficult to remove from the wafer surface. For instance, a wet scrubbing method by using a brush cannot remove the haze from the wafer surface. The nitride haze acts as an additional insulating layer on top of the silicon wafer and presents processing difficulties in subsequently carried out processes. One of such processing difficulties occurs in the formation of LOCOS oxide insulation. The nitride haze impedes the growth of LOCOS oxide. Similar contaminants have also been observed in a TEOS oxide deposition process with similarly undesirable results.
It is therefore an object of the present invention to provide an apparatus for preventing contamination to a process chamber that does not have the drawbacks or shortcomings of a conventional apparatus.
It is another object of the present invention to provide an apparatus for preventing contamination to a process chamber used in a low pressure chemical vapor deposition process that includes an exhaust-vent device for venting the chamber during wafer loading and unloading steps.
It is a further object of the present invention to provide an apparatus for preventing contamination to a low pressure chemical vapor deposition chamber which includes an exhaust-vent device connected parallelly with a gate valve such that the process chamber may be continuously evacuated during wafer loading and unloading steps.
It is another further object of the present invention to provide an apparatus for preventing particle contamination to a low pressure chemical vapor deposition chamber by using an exhaust-vent device connected parallelly with a gate valve for providing continuous evacuation of the chamber wherein the device is equipped with a reduced cross-sectional area for increased fluid flow and reduced probability of particle depositions.
It is still another object of the present invention to provide an apparatus for preventing contamination in a low pressure chemical vapor deposition chamber by utilizing an exhaust-vent device connected for bypassing a gate valve which has a section of reduced cross-sectional area of less than one half of the remaining sections.
It is yet another object of the present invention to provide a method for preventing contamination to a low pressure chemical vapor deposition chamber by connecting an exhaust-vent device to bypass a gate valve and then turning on the device during loading and unloading of wafers from the chamber.
It is yet another further object of the present invention to provide a method for preventing contamination to a low pressure chemical vapor deposition chamber by using an exhaust-vent device equipped with a first and second conduit each having an internal diameter at least two times that of a middle conduit such that a fluid flow rate through the middle conduit is greatly increased for preventing particulate deposition in the middle conduit.